Your position is definitely a difficult one (especially have missed a unit of the subject), but if you've already exhausted all your normal avenues (seeing you teacher for extra help, questions thread on atarnotes, online tutorials, etc.) it's a matter of not giving up hope in the subject. Certainly, methods can be a very difficult subject. I personally struggle a lot with many worded questions, and there are so many people that just seem to get it instantly. That being said, I wouldn't adopt the attitude that because you know it'll be a bottom 2 subject that you stop trying. If you do all your set homework and work on the areas you know your struggle with, I don't see any reason why you shouldn't be able to achieve your 20. Remember that the bell curve works both ways, too. Best wishes!
edit: You posted midway through, but I wouldn't worry too much about your unit 3 grade. methods is a subject more reliant on your exam performance anyway! The crux of what I'm trying to say above essentially boiled down to not adopting a negative attitude, which I'm sure you're capable of overcoming if you are doing well in your other subjects like you said! Find your motivation and don't expect to be able to get every question right. From now until the exam, it doesn't matter at all how many mistakes you make and for every one that you do, it is simply a learning experience of one less you'll make at a later point.